Ferroli A37 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A37 fault code mean?
The A37 fault code indicates that your Ferroli boiler's pressure sensor has detected the central heating system pressure has dropped to an unsafe level — typically around 0.4 bar or below. At this point the boiler locks out automatically to protect itself from running dry. The fix can be as straightforward as topping up the system pressure, but if pressure keeps dropping the underlying cause — almost always a leak somewhere in the system — needs to be properly diagnosed and repaired. Note: fault code A23 also relates to water pressure loss on Ferroli boilers and shares many of the same causes and fixes described on this page.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- System pressure too low due to a leak Common
A slow drip from a radiator valve, towel rail valve, or pipework joint is the most common reason system pressure gradually falls until the A37 is triggered. The leak may be small enough that you never see a puddle — check all radiator valves, towel rail connections, and visible pipework for damp patches or corrosion staining.
- Air trapped in radiators Common
If the system hasn't been bled recently, air pockets can give the pressure sensor a false low-pressure reading and also cause the system to lose pressure over time. Bleeding the radiators with a bleed key is a straightforward DIY task.
- Under-pressurised or failing expansion vessel Sometimes
The expansion vessel absorbs pressure changes as the system heats and cools. If its internal diaphragm has failed or its pre-charge pressure is too low, the boiler's pressure can drop when the system cools down. This is a common cause of recurring A37 faults even after topping up.
- Faulty or failing pressure sensor Sometimes
If the wiring, connectors, or the sensor itself are damaged, the boiler may report low pressure even when the actual system pressure is normal. An engineer can check the DC voltage output of the sensor (expected range 4.9–5.1 V DC) to confirm whether it needs replacing.
- Leaking internal auto air vent or safety relief valve Sometimes
The auto air vent inside the boiler casing and the 3-bar pressure relief valve can both weep slowly, causing gradual pressure loss with no obvious external leak. The relief valve may also discharge if pressure has repeatedly been over-boosted during top-ups.
- Cracked or corroded heat exchanger Rare
A hairline crack in the heat exchanger's helical pipe can allow water to escape internally. This is less common but should be suspected if no external leak can be found and pressure loss continues after all other checks.
How to fix it
- Check the pressure gauge and top up via the filling loop DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, locate the filling loop (usually a silver braided hose with two valves beneath the boiler). Slowly open both valves until the gauge climbs to around 1.2 bar, then close the valves fully. The boiler should clear the lockout once pressure rises above 0.8 bar. Reset the boiler using the reset button — consult your user manual for the exact method.
- Bleed the radiators to remove trapped air DIY safe
With the system off and cooled down, use a radiator bleed key on each radiator starting with the ground floor and working upwards. Hold a cloth beneath the bleed valve and open it a quarter-turn until water (not air) flows steadily, then close it. After bleeding all radiators, re-check the pressure gauge and top up again if needed, as bleeding typically drops system pressure slightly.
- Inspect all radiators, valves, and visible pipework for leaks DIY safe
Work around every radiator in the house and look closely at the valve bodies, valve tails, and pipe connections for any sign of moisture, mineral staining, or corrosion. Even a slow weep around a packing nut can cause the A37 to recur. If you find a dripping valve, you can try gently tightening the gland nut — but do not force it. Any actual pipe or component leak repair must be carried out by a qualified engineer.
- Monitor pressure over 24–48 hours after topping up DIY safe
If the pressure holds steady at 1.0–1.5 bar with no further drop, the issue may have been a one-off. If it drops again within a day or two, there is an active leak or a failing expansion vessel that requires professional attention. Keep a note of how quickly pressure falls — this helps an engineer narrow down the cause quickly.
- Have a Gas Safe engineer inspect the expansion vessel Gas Safe engineer
An engineer can check the expansion vessel's pre-charge pressure using a Schrader valve gauge and re-pressurise or replace it if needed. A failed expansion vessel diaphragm is a very common cause of recurring pressure loss and is not something a homeowner can safely diagnose or repair. Replacement is straightforward for an engineer and typically takes under an hour.
- Have a Gas Safe engineer test the pressure sensor and wiring Gas Safe engineer
If no leak or expansion vessel fault is found, an engineer should check the pressure sensor's electrical output (the expected DC voltage is in the 4.9–5.1 V range) and inspect the sensor wiring and connectors for damage or corrosion. A faulty sensor is replaced as a unit and is a straightforward repair.
- Have a Gas Safe engineer carry out a full leak investigation if pressure loss continues Gas Safe engineer
If the source of pressure loss still hasn't been identified, an engineer can pressurise the system and carry out a systematic check including the boiler's internal auto air vent, the 3-bar safety relief valve discharge pipe, and — as a last resort — the heat exchanger. A leaking heat exchanger is rare but means the heat exchanger must be replaced, which is a major repair.
Parts you may need
- Water pressure sensor · from £35
- Expansion vessel (replacement) · from £55
- Expansion vessel Schrader valve core · from £5
- Radiator bleed key · from £3
- Filling loop assembly · from £20
- Pressure sensor wiring loom · from £25
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£300, depending on the underlying cause.
Frequently asked questions
Why does my Ferroli boiler keep losing pressure and showing A37 even after I top it up?
If the pressure drops again within a few days of topping up, there is almost certainly an active leak or a failed expansion vessel. A one-off pressure loss can happen after bleeding radiators or following a power cut, but recurring A37 faults mean the root cause hasn't been addressed. A Gas Safe engineer can trace the leak — which is sometimes inside the boiler casing — and either repair the leak or replace the expansion vessel. Simply topping up repeatedly is not a permanent fix and can overload the pressure relief valve over time.
Can I top up my Ferroli boiler pressure myself to clear the A37?
Yes — re-pressurising via the filling loop is a straightforward DIY task and is the correct first step. Aim for 1.0–1.5 bar on the pressure gauge with the system cold. However, if the A37 returns, you should call a Gas Safe engineer rather than keep topping up. Repeated top-ups without fixing the underlying cause can introduce excess water and air into the system, accelerating corrosion.
What is the difference between fault codes A23 and A37 on a Ferroli boiler?
Both A23 and A37 relate to low system water pressure on Ferroli boilers, and the causes and fixes are essentially the same — the boiler's pressure sensor has detected pressure below a safe operating threshold. The slight difference is in which internal monitoring path triggered the lockout, but from a homeowner's perspective the diagnostic steps are identical: check pressure, top up if low, look for leaks, and call an engineer if it recurs.
How much does it cost to fix a Ferroli A37 fault in the UK?
For most people the repair costs between £100 and £300 including labour. A pressure sensor swap typically costs around £80–£150 all-in, and expansion vessel replacement usually falls in the £120–£200 range. Leak repairs vary — a straightforward valve repack or fitting replacement is often under £150, but if the leak turns out to be from the heat exchanger, replacement costs can run to £400–£600 or more. If your Ferroli is over 10 years old and facing a major component repair, it is worth comparing the repair cost against a new boiler installation.